For such an important character, we don't know much about Eppie. But we do know that we love her. Even as a little girl, she's adorable. She affectionate and trusting—she latches on to Silas right away—and she's full of high spirits and lovable mischief. The narrator describes her as "a creature of endless claims and ever-growing desires, seeking and loving sunshine, and living sounds, and living movements" (1.14.33); she loves flowers and butterflies, and birds and animals.
Basically, she's the exact opposite of Silas.
Not much changes in sixteen years. When we meet Eppie again, she's still high-spirited and full of mischief, talking about her desire for a garden just where Aaron Winthrop will hear, because she knows that he'll volunteer to dig it for her. She has a little dog and cat that she frolics with, and she calls Silas by pet names.
The one serious spot in Eppie's nature, and really the only thing that makes her tolerable, is that she's completely devoted to Silas. She doesn't even want to marry Aaron, because she's afraid that she'll have to leave her father. And when Godfrey offers to make her a lady, she refuses quickly and firmly. For all her light-hearted girlishness, Eppie is, by the novel's standards, going to be a good woman.